Most of known humidity sensors are based on the measurement of capacitance changes. In other words, the sensing of humidity is done by measuring changes in capacitance value. Most of such capacitive type humidity sensors use a porous alumina film or a porous polymer film. Generally, the changes in the capacitance value are not so large. Moreover, the changes are non-linear for a wide range of 10 to 90% relative humidity (RH) change. Further, a complex circuitry is required for detecting the small changes and for linearization of the detected signals. Due to this complex circuitry, response or sensing time is high for the capacitive type humidity sensors. In resistive type known humidity sensors, main problem is to measure high resistance of the order of GΩ at 10% RH. Porosity is one of the most important parameters for sensing humidity, but it also increases the resistance of the humidity sensor. Thus, such a high resistance limits the sensing range at high 50 to 100% RH. Further, it is also very difficult to develop electronic circuitry to measure such a high resistance.
Patent publication numbered US20100031745 A1 describes a nano porous humidity sensor, and humidity sensing based on change in capacitance. In this sensor, it requires an electrically conductive adhesion layer to produce a robust anodically anodized oxide film on the substrate. All the processing and deposition are done in class-1000 clean room. Further, a disturbance is detected in capacitance of electric current applied across the nano-sensor by a power source. For this purpose, the presence of water vapor in air is detected. The relative humidity of air is determined by analyzing an output electrical signal that is proportional to a concentration of oxidizing or reducing agents on the metal surface of the nano-sensor. The output electrical signal is strongly dependent on the concentration of oxidizing or reducing agents present in air. Thus, accurate detection of water vapor is so critical in any chemical and dusty environment. The detection of concentration of oxidizing or reducing agents present in air is also critically dependent on nano-porous structure of alumina by anodization. All the processing steps of nano-sensor require a class-1000 clean room that adds to the cost of the sensor. The other disadvantage is that the detection capability of the nano-sensor can vary with non-porous distribution. Further, the repeatability of the nano-sensor may vary with time and processing steps.
Patent publication numbered WO2007016681 A2 describes a humidity sensor in for of a deflectable resistor comprising of a substrate and a first layer of a conductive material. Any moisture contacting the surface of the humidity sensor penetrates a number of cracks in a first layer of conductive material. The space between the cracks in the first layer of the conductive material fills with moisture and thereby resistance decreases as the amount of moisture content increases. In another alternative arrangement, the substrate is bendable between a first configuration and a second configuration. The resistance measured between the first end and the second end of an electrically conductive ink layer, which changes by electrical signal applied on it, predictably when layer is bent. The change of resistance of the layer of the conductive ink reflects an amount of deflection between the first configuration and the second configuration. The basic disadvantage of this type of humidity sensor is that moisture content measurement is dependent upon micro-cracks in the conductive material, which can be contaminated easily by dust, smoke and temperature conditions. This can drift the resistance values and may give erroneous results.
U.S. Pat. No. 6,342,295 B1 describes a moisture sensor comprising of a pair of electrodes and a moisture sensitive film deposited between conducting particles dispersed in a hygroscopic polymer. The hygroscopic polymer is a polymer comprising the polyether amine, the epoxy compound, and a water-soluble nylon or a mixture of these polymers that starts to absorb moisture in a humidity region 60% to 90% RH. The disadvantage of such a moisture sensor is polymerization of the polyether amine, the epoxy compound, and the water-soluble nylon is a typical process. Another disadvantage is the amount of conducting particles loading to a critical parameter without which the moisture film loses its sensitivity. The loading of conducting particles in the moisture film is also a typical process. These lengthy and typical synthesis processes make the moisture sensor cost ineffective. In addition, moisture sensitive polymer has limitation towards contamination and thermal stability. Further, the moisture sensitive polymer has a limited range of sensitivity 60 to 90% RH.
U.S. Pat. No. 5,136,274 A describes the detection of relative humidity as a change in electrical resistance using a porous sintered body and electrodes into contact with solution of a polyurethane resin. Further, a durability test of the humidity sensor is conducted. In this durability test various tests, such as water resistance test, oil test, Organic acid test, Inorganic corrosive gas test, cigarette smoke test, and aldehyde test are conducted at fixed 60% relative humidity. The main disadvantage is the variance of base resistance with different porous sintered body compounds. Such a humidity sensor cannot be operated at higher temperatures due to coating of urethane resin.
U.S. Pat. No. 4,635,027 A describes a resistance-variation type moisture sensitive film made essentially of Sodium Styrenesulfonate 100 parts by weight and 3 to 7 parts of polymer. A moisture sensitive film of a polyelectrolyte detects variation of humidity by means of a variation in resistance. The main disadvantage of electrolytic and polymer materials are that they are hydrophilic and soluble in water. Therefore, they have a poor durability against water or dew condensation. Another disadvantage of this moisture sensitive electrolyte is that it is chemically active and unable to operate in the presence of ammonia or organic solvents. Further, thermal stability of this material also restricts its operation at high temperatures.
U.S. Pat. No. 4,484,172 A describes a semiconducting device comprising a humidity sensitive metal oxide coating of lanthanide series cobaltite. An amplifier in conjunction with a humidity detector film is provided that ignites an electric filament lamp or signaling a glow lamp. This device detects changes in resistance from 30 to 100% RH based on film's thickness. The resistance response of the humidity sensor becomes constant after 70% RH which otherwise exhibits its sensitivity between 30 to 70% RH. Another disadvantage is that the detecting film comprised of lanthanide series elements, which makes the product very expensive.
U.S. Pat. No. 4,447,352 A describes the detection of humidity by varying electrical resistance of a humidity sensitive element. The humidity sensitive element consists of 70 to 20% Zinc Ferrite and 30 to 80% a selected group of MnO2 and SiO2, which reduces specific resistance of metal oxide, and thereby detects humidity by changing the electrical resistance. A thin film electrode of Ag, Au, and RuO2, is prepared on both sides of the said material either by printing or by vapor deposition technique. The stability and repeatability data of the humidity sensitive element is not provided. Further, the details of electrode formation technique being typical in nature have not been dealt with. Further, the response time measurement of the humidity sensitive element is also not provided.